Summary:
This adds basic support for mutable history for arc diff and arc amend for
mercurial. This is purely opt-in (so it shouldn't affect anyone who doesn't want
this feature) by explicitly setting
"immutable_history" : false
in arc configuration.
This also fixes another instance of weird behaviour for multiple heads - the
first instance was fixed here:
https://github.com/facebook/arcanist/pull/28
Test Plan:
without "immutable_history" turned on)>
When ##arc diff## produces an update diff, it should list only commits that are
ancestors of the current revision - not ones from other heads.
Reviewers: epriestley
CC: csilvers, aran, Koolvin
Differential Revision: https://secure.phabricator.com/D2654